IGRP Metrics

IGRP Metrics
Metrics are the mathematics used to select a route. The higher the metric associated with a
route, the less desirable it is. For IGRP, the Bellman-Ford algorithm uses the following equation
and creates the overall 24-bit metric assigned to a route:
metric = [(K1
×
bandwidth) + [(K2
×
bandwidth)
÷
(256 – load)] + (K3
×
delay)]
×
[K5
÷
(reliability + K4)]
The elements in this equation are as follows:

By default, K1 = K3 = 1, K2 = K4 = K5 = 0. Therefore, by default, the metric formula
reduces to:
metric = (1
×
bandwidth) + (1
×
delay)
metric = bandwidth + delay

The
show ip protocols
command shows you the configured K-values for all IGRP and
EIGRP autonomous systems. Notice from the following output that the default values are
set for IGRP AS 100:
Router#
show ip protocols
Routing Protocol is "igrp 100"
Sending updates every 90 seconds, next due in 71 seconds
Invalid after 270 seconds, hold down 280, flushed after 630
Outgoing update filter list for all interfaces is
Incoming update filter list for all interfaces is
Default networks flagged in outgoing updates
106
Chapter 4 
IGRP and EIGRP
Default networks accepted from incoming updates
IGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
IGRP maximum hopcount 100
IGRP maximum metric variance 1
Redistributing: igrp 100
Routing for Networks:
10.0.0.0
192.168.24.0
Routing Information Sources:
Gateway Distance Last Update
10.10.10.1 100 00:00:49
Distance: (default is 100)

Delay is computed as one-tenth the sum of all the measured delays, in microseconds, of the
outbound interfaces of the links along the path, which is to say, the cumulative delay along
the path in tens of microseconds.

Bandwidth = [10000000/(BW in Kbps)]. BW is the lowest bandwidth of the links along
the path.

You can compute delay yourself and find the lowest bandwidth in the path by issuing the
command
show interface
interface_type interface_number
on each outbound
interface along the path, adding up the
DLY
values and comparing bandwidths to determine
the lowest. Here’s a sample of the output for the Serial 0/0 interface, with all but the first
four lines of output removed:
Router#
sh int s0/0
Serial0/0 is up, line protocol is up
Hardware is PowerQUICC Serial
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Here you can see this interface is showing an MTU of 1,500 bytes, a bandwidth of 1,544 Kbps
(1.544 Mbps, or the T-1 rate), a delay of 20,000 microseconds, reliability of 100 percent, and
the lowest possible transmit and receive loads (1/255). The actual metric computation, based on
cumulative measured values, is demonstrated in the section “Verifying and Troubleshooting
IGRP” later in this chapter.
The preceding formula is used for the non-default values of K5, when K5 does
not equal 0. If K5 equals the default value of 0, then that part of the formula is not
used at all. This formula is used instead: metric = [(K1
×
bandwidth) + [(K2
×
bandwidth)
÷
(256 – load)] + (K3
×
delay)]. The important point is that the final
term—[K5
÷
(reliability + K4)]—would evaluate to 0 when K5 is set to 0, making
the entire metric 0. This would be unacceptable, which is why the term is omitted
when K5 equals the default value of 0.
Interior Gateway Routing Protocol 107
If necessary, you can adjust metric constants in router configuration mode. Metrics are used
to change the manner in which routes are calculated. After you enable IGRP on a router, metric
weights can be changed using the following command:
metric weights tos K1 K2 K3 K4 K5
Note that the no version of this command returns the constants to their default values, and that
the type of service parameter (tos) must always be 0, as shown in the following router output:
Router(config)#router igrp AS_number
Router(config-router)#metric weights ?
<0-8> Type Of Service (Only TOS 0 supported)
Table 4.2 shows the relationship between the constant and the metric it affects.